CLEANUP: session: use local variables to access channels / stream ints

CLEANUP: session: use local variables to access channels / stream ints

In process_session, we had around 300 accesses to channels and stream-ints
from the session. Not only this inflates the code due to the large offsets
from the original pointer, but readability can be improved. Let's have 4
local variables for the channels and stream-ints.